2-Row is a variety of barley with 2 rows of grains on the head. Barley has a 6-fold symmetry in the head formation, but in 2 row only 2 of the 6 form full grains the other 4 fail to develop beyond the flowering stage. 2-row tends to have a larger grain size than 6-row barley.
I know the difference has to do with how the grains grow on the stock, but I'm not completely sure what that difference is. It also seems like 2-row is more commonly used as a base malt, but I've seen ...